Influenza is an enormous public health threat worldwide, resulting in up to 500,000 deaths annually and manifold more during pandemics. Secondary bacterial pneumonias are a major fatal complication of influenza, through mechanisms which remain poorly elucidated. The long-term goal of my research is to unravel the mechanisms responsible for influenza-induced immunosuppression of antibacterial host defense in the lung, with the immediate objective of this proposal being to examine the role of type I interferons (IFNs) and IL-27 as critical mediators of neutrophil responses during post-influenza pulmonary infections by Streptococcal pneumoniae, which is the most significant cause of bacterial pneumonia worldwide. Our published data show that type I interferons (IFNs) induced during influenza infection suppress early neutrophil responses against secondary S. pneumoniae infection, resulting in impaired bacterial clearance and increased mortality. This is attributable to type I IFN-mediated suppression of CXCR2 ligands, KC (CXCL1) and MIP-2 (CXCL2), which are critical to neutrophil recruitment, and we furthermore show that early restoration of these chemokines reverses the defects in bacterial clearance following influenza. Our subsequent studies reveal that IL-27, a novel member of the IL-12 family of heterodimeric cytokines, is an IFN-regulated gene that is induced during influenza infection and acts as a negative regulator of KC and MIP2 production and neutrophil recruitment during post-influenza bacterial pneumonia. We therefore hypothesize that during influenza infection, type I IFNs mediate impairment of neutrophil responses against secondary S. pneumoniae infection by inducing the expression of IL-27. To test this hypothesis, we will perform studies with the following aims: 1. To determine the role of IL-27 in type I IFN-mediated susceptibility to post-influenza bacterial pneumonia; and 2. To determine the mechanisms by which IL-27 suppresses neutrophil responses. Using gain-of-function and loss of function approaches, the proposed studies will examine the in vivo role of IL-27 as a mechanism through which IFNs inhibit early neutrophil responses and impair bacterial clearance. Furthermore, we will examine the direct mechanisms through which IL-27 modulates neutrophil responses against post-influenza bacterial pneumonia using a combination of in vivo and in vitro approaches. The results of these studies will provide important mechanistic insights into how early neutrophil responses are impaired during post-influenza bacterial pneumonias, as well as further our understanding of the role of type I IFNs during bacterial infections. Furthermore, since little is known about the role f IL-27 in innate immunity and infection, the studies proposed are likely to advance the field by providing new information about how IL-27 regulates neutrophil responses, particularly in the context of a highly important clinical problem. Finally, given the ongoing global impact of influenza infections, the results of these studies will determine whether IL-27 is an IFN-regulated molecule that may be therapeutically manipulated to reverse influenza-induced impairment of pulmonary host defense.